Not applicable.
The invention relates to apparatus and techniques for providing a phased array radar system having a blanker for reducing the effects of sidelobe interference which is capable of effective operation even in the presence of strong sidelobe jamming.
As is known in the field of radar systems, jamming is a technique used to interfere electronically with an enemy radar system. The jammer generates and transmits radar signals which are designed to saturate or otherwise effectively disable the enemy radar receiver.
Jammers may generate jamming signals based on knowledge of typical radar operating bands of frequencies of the system to be jammed. The jammer signal may be transmitted over a limited frequency interval based on knowledge or an estimate of the instantaneous frequency of the jammed radar system (this technique is sometimes referred to as repeater jamming). Smart jammers have the capability to determine the frequency of the transmitted radar signals and focus the jamming energy over a narrow band at the determined frequency (this technique is sometimes referred to as spot jamming). Another type of jamming is referred to as barrage jamming in which jamming signals are radiated over a wide band of frequencies
Various techniques are utilized to counteract the effects of interference signals, including jammer signals, in sidelobes of the main, or sum beam. One such technique is referred to as sidelobe canceling, in which the main beam sidelobes are minimized and preferably, nulled. Sidelobe cancelers can also be used to provide a null in a main beam sidelobe in the direction of the jammer. To this end, auxiliary antenna(s) (typically one antenna element per jammer) receive and process jammer signals in order to determine weights necessary for the auxiliary antenna outputs to be added to the main beam in order to provide a null in the sidelobes at the jammer direction.
Preferably, auxiliary antennas used for side lobe cancellation have a relatively wide field of view, as may be provided by an omnidirectional, or isotropic antenna. However, such antennas have relatively low gain such as on the order of 6 dB, requiring significant gain to be introduced in order to effectively null the sidelobes in the direction of the jammer. Consider for example, a main beam pattern in which the peak sidelobe has a gain of +20 dBi, and (which corresponds to the rms error sidelobe level of a well designed antenna array). For a jammer in a xe2x88x9215 dBi sidelobe, the auxiliary channel is attenuated by +6xe2x88x92(xe2x88x9215)=21 dB. However, a jammer in a +20 dBi sidelobe requires 14 dB amplification of the auxiliary channel (20 dBi=+6+14). The introduction of such gain on the auxiliary channels amplifies thermal noise, which couples into the main beam, thereby degrading, the sensitivity of the main beam. Furthermore, nulling one sidelobe tends to increase sidelobes in other directions. Even with these drawbacks, however, the sidelobe canceler can be an effective way of counteracting the effects of sidelobe jamming.
Blanking is another technique used to reduce the effects of sidelobe interference. A blanker utilizes a dedicated receive antenna and processing channel, with the signal processing being matched to the main beam processing. The gain of the blanking channel is greater than that of the main beam sidelobes. The outputs of the main beam processor and the blanker processor are compared in order to determine whether main beam target detections are valid. More particularly, if sidelobe interference is strong, then the signal strength in the blanker channel will be stronger than in the main beam channel and the main beam output is rejected. Alternatively, if the signal strength in the main beam channel is greater than that in the blanker channel, then main beam target detections are considered valid. Interference signals blanked in this manner include strong sidelobe clutter, larger aircraft, repeater jammers, or jammers which radiate radar signals like chirp signals.
However, one problem with blankers is the requirement that its antenna gain be greater than the sidelobe gain. As with the sidelobe canceler, the output signal of the blanker must be amplified to blank a sidelobe having more dBi gain than does the blanker. With such high amplifier gain, system noise levels are amplified which results in degradation of main beam sensitivity. Furthermore, in the event that the blanker antenna is spaced from the main beam aperture, multipath reflections can reduce the strength of incoming signals in the blanker while increasing them in the main beam, thereby negatively affecting blanker performance. As a result of these drawbacks, blankers are sometimes disabled in the presence of strong sidelobe jamming.
Because blankers and sidelobe cancelers are useful to reduce the effects of different kinds of jamming, and also because each is not without its problems, particularly the exacerbation of noise, generally one such technique will be operated at a time. That is, in the presence of sidelobe jamming, the canceler is used to introduce a null in the direction of the jammer and the blanker is turned off; whereas, in the presence of repeater jamming, the blanker is used and the canceler is turned off.
An additional feature of some radar systems is a spoofer used to reduce the effectiveness of smart jammers. Smart jammers are capable of listening to a transmitted radar signal to deduce its frequency and then focusing the jamming energy to a fraction of the radar band at the deduced frequency. A spoofer includes a waveform generator which generates a spoofer signal capable of confusing a smart sidelobe jammer so as to prevent the jammer from ascertaining the frequency of the actual radar signals. That is, the spoofer signal is selected to camouflage the actual radar signal. This may be achieved in various ways, such as by transmitting a noise-like signal or by transmitting a replica of a radar signal. As one example, the spoofer signal may be a swept sinewave having an amplitude greater than the amplitude of the actual radar signal over the entire frequency range of operation. While it is desirable that the spoofer antenna be nearly omnidirectional, it is also desirable that the spoofer power be relatively low.
Spoofers typically utilize a separate transmit antenna spaced from the main transmit antenna. This is because antiradiation missiles can lock on xe2x80x9cactive decoysxe2x80x9d which are radar signals radiated in frequencies surrounding the frequency of the actual radar signal. If the spoofer antenna were integral to the main transmit antenna, its use could cause an antiradiation missile to lock onto the radar system.
In accordance with the present invention, a radar system includes an antenna for receiving radar signals, said antenna including a main antenna having a main beam pattern and a blanker antenna having a blanker beam pattern and a beam forming network including a nulling circuit, said beam forming network being coupled to said main antenna for forming a sum beam having a null in the direction of a jammer and for forming a blanker beam having a null in the direction of the jammer. The radar system further includes a first signal processor for processing radar signals received in said sum beam; a second signal processor for processing radar signals received in said blanker beam; and a comparison circuit for comparing the level of signals received in said sum beam with the level of signals received in said blanker beam in order to determine when radar signals received in said main beam are representative of a valid target. With such an arrangement, a radar system can operate in the presence of strong sidelobe jamming without the conventional problems attributable to noise amplification.
In accordance with another feature of the present invention, the radar system further includes an open loop ECM nulling map circuit coupled to said beam forming network for detecting the direction of the jammer, wherein the nulling circuit is responsive to the detected jammer direction by providing a null in the sum beam pattern and in the blanker beam pattern. The radar system also includes a plurality of auxiliary antennas and a sidelobe canceler coupled to said plurality of axiliary antennas. With such an arrangement, certain drawbacks of the sidelobe canceler are overcome with the sidelobe canceler providing additional jamming rejection. More specifically, with the open loop nulling, the sidelobe auxiliary antennas, will have more gain in the jamming direction than the main beam, rather than less gain as would be the case in absence of open loop nulling in the jammer direction.
This is achieved with the use of an open loop nulling circuit for introducing one or more nulls into the blanker and main beam patterns in the direction of the jammers. The main beam patterns include a sum beam (beams) and difference channels, such as xcex94Az, xcex94El, and xcex94xcex94. That is, whereas, conventional blankers were disabled in the presence of strong jamming in order to prevent excessive noise from being coupled into the main beam, by minimizing the sensitivity of the blanker channel to sidelobe jammer signals, the blanker can be kept on in the presence of strong sidelobe jamming. Stated differently, by providing the blanker with nulls in the jammer directions but with high gain elsewhere, the effectiveness of the blanker as to sidelobe interference and barrage jamming is maintained while reducing its sensitivity and thus also the noise issues associated with blanker operation in the presence of jamming.
Furthermore, with this arrangement, omnidirectional sidelobe canceler antenna(s) can be used without the conventional problems associated with canceler operation in the presence of strong sidelobe jamming. That is, whereas it has heretofore been necessary to introduce significant gain into the canceler channels in order to effectively cancel the sidelobes in the presence of strong jamming, since the sidelobes are already reduced by the open loop nulling in the main and blanker beams, the canceler gain necessary to further reduce the effect of the jamming signals and to cancel the sidelobes is reduced. Stated differently, by introducing nulls into the main and blanker beams, the work of the sidelobe canceler is made easier (i.e., less gain is required), thereby reducing the problems associated with noise amplification. Thus, the effectiveness of the canceler as to sidelobe jamming is maintained, but without the conventional noise problems associated with canceler operation in the presence of sidelobe jamming.
In accordance with another feature of the present invention, the radar system includes an antenna having a pair of linear arrays disposed orthogonal with respect to one another. With such an arrangement, one of the linear arrays provides full azimuth coverage (i.e., is omnidirectional in azimuth) but limited elevation coverage, such as on the order of 10 degrees. The second linear array is omnidirectional in elevation with adaptive gain in azimuth in order to point beams in the direction of the jammers. The difference beam generated by subtracting signals received by the second linear array from signals received by the first linear array provides a composite beam that is omnidirectional in azimuth and has nulls in the direction of the jammers.
With this arrangement, the blanker beam has xe2x88x9215 dBi nulls in the direction of the jammers and thus, is able to operate even in the presence of strong sidelobe jamming. Further, the canceler auxiliary antennas have 21 dB more gain in the jamming directions than the blanker beam, instead of 14 dB less (for a 20 dBi sidelobe in the jammer direction).
In one embodiment in which the radar system farther comprises a spoofer, the first linear array provides the blanker antenna on receive and the spoofer antenna on transmit. Use of the null(s) in the antenna beam patterns advantageously reduces the power associated with the spoofer, since the power required of the spoofer is less in a xe2x88x9215 dBi main beam than for a +20 dBi sidelobe. Further, the nulling permits the spoofer antenna to be integral with the main beam aperture without adding significant risk of antiradiation seekers locking onto the spoofer signals.